(1) Field of the Invention
The invention relates to a method for texturing virtual three-dimensional objects, particularly virtual three-dimensional building objects and city models.
(2) Description of Related Art
With respect to a graphical representation of a virtual three-dimensional object generated by a computer, the texture or texturing, respectively, is the image displayed on the surface of the three-dimensional object. In a digital textured 3D modeling used for the planning of buildings or cities, for example, façade surfaces are displayed on the surface of the objects, which represent views of the façades of real objects, i.e. real houses or other buildings, with a certain recognition value.
For texturings of this type, two methods are currently available. In a first method, a photorealistic texturing is performed. A photographic image of the surface of the real object is projected onto the surface of the virtual object. As a rule, picture editing methods are used for this purpose. In such a method, many details of the real object are reproduced due to the use of real image data, so that the virtual three-dimensional object allows a high informative content.
However, this method requires a high image quality of the photographic source material, which may not be guaranteed from the beginning and, depending on the image material, may be difficult to correct subsequently. In general, such a texturing also requires a high degree of manual processing work, during which the textures of the virtual objects are checked for correctness one by one and the real photographic image data have to be adapted to the virtual object, especially scaled and, if necessary, freed from faults. Due to the use of real image data the generated virtual objects require a great data volume, and even if the representation of the object is scaled down, all image information of the original image material have to be stored together with the virtual object.
As an alternative, also a generic texturing may be applied. The graphical elements are positioned on the surface of the virtual object in the form of a schematic representation of the real object, so that the entire surface of the virtual model is generated artificially. Such a representation reproduces fewer details of the real object. The so achieved informative content is much smaller than that of a photorealistic representation. It is, however, an advantage of such a method that already prefabricated graphical elements are used according to the modular design principle, so that a high degree of automation can be achieved. Also, the generated virtual objects are significantly smaller as far as their data volumes are concerned as compared to the photorealistically textured 3D objects.
In the practical use of virtual 3D objects, where it should, on the one hand, be possible to reproduce and process the three-dimensional model with as many details as possible while keeping the storage capacity low, it shows very frequently that there is no practicable method to ensure a great richness of details for the 3D objects on the one hand, and a low data volume on the other hand. In many cases, the optimum reproduction of a real object in the virtual model is exactly at a point somewhere between the photorealistic and the generic representation, a virtual object need not reproduce all details of the real object, but the purely generic representation of the object is too rough and short of details.
Also, both texturing methods require in many cases a very considerable manual expenditure. Depending on the complexity of the object in the specific case, the texturing may constitute up to 60% of the overall expenditure of a project. In typical projects, where three-dimensional city models are generated on the computer, currently—despite highly effective individual solutions in some cases—a great amount of work has to be accomplished manually, so that thousands of man hours of highly specialized technical personnel may be necessary.